An oil viscosity is very important among physicochemical properties of oil. Further, monitoring an oil viscosity allows for detecting change in oil status. If the oil is timely replaced with the change in oil status, then the service lives of mechanical systems can be prolonged and their downtime can be also shortened. Moreover, a maintenance cost of the mechanical systems can decrease and a replacement period for replacing the oil can be extended.
Change in the oil viscosity may precede before critical problems occur in the mechanical systems. Thus, if the change in the oil viscosity could be detected beforehand, then detailed causes of the change in the oil viscosity would be investigated through a laboratorial analysis. Generally, the oil viscosity may increase due to oxidation of the oil, penetration of mixture, generation of air bubbles, etc. Further, oil deterioration may advance along with those factors.
It is well known to those of ordinary skill in the art that the oil viscosity is measured by the laboratory analysis. However, it is difficult to develop a reliable and economical viscosity sensor for measuring the oil viscosity in an on-line or in-line manner.
Monitoring the oil viscosity in real time allows for detecting the following before the mechanical system fails: whether improper oil is used in the mechanical system; whether moisture is mixed into the oil; and whether oil is contaminated with oil-deteriorating products. Further, obtaining data on the oil viscosity in real time eliminates the inconvenience of periodically sampling the oil and measuring the oil viscosity by the laboratory analysis. In particular, human errors associated with sampling the oil can be prevented. Thus, demands for measuring the oil status in real time have recently increased.
An in-line viscometer measures the viscosity of the oil that is used in a tribo system. Thus, the in-line viscometer must have a long service life to work continuously in an aggressive environment (e.g., under conditions of high temperature, high pressure and intensive vibration). Further, the in-line viscometer must provide information on a replacement period and a service life of the oil by reliably measuring a current status of the oil state and then judging the measured values. Furthermore, the in-line viscometer must be compact, economical and precise.
Historically, the in-line viscometer such as a capillary tube viscometer, a rotary viscometer and a falling piston viscometer was developed. However, such in-line viscometers have a complicated structure since they use the measure principle employed in a laboratorial viscometer as it is. Accordingly, new methods and studies for solving such a problem have been devised and made in a worldwide scale. By way of example, there exist in the art a vibrational viscosity sensor and an acoustic viscosity sensor.
The vibrational viscosity sensor includes a rod, a cylinder and a movable sensor part of a fork or tube shape to vibrate at a resonant frequency. In the vibrational viscosity sensor, a viscosity of fluid acts as a force for damping the vibration. Generally, as the viscosity increases, the amplitude and bandwidth of the damped vibration decrease and increase, respectively. As examples of the vibrational viscosity sensor, a twisting viscometer and a tuning fork viscometer are disclosed in U.S. Pat. Nos. 4,811,593 and 7,043,969, respectively. There are disadvantages in that such vibrational viscometers have a relatively large size and the movable sensor part must be sealed.
The acoustic viscosity sensor has advantages in that it is sized relatively small and does not have a movable sensor part. The acoustic viscosity sensor excites a compression wave, which is referred to as the so-called acoustic wave, into fluid and measures a wave reflecting from the fluid. An example of the acoustic viscosity sensor is disclosed in U.S. Pat. No. 6,439,034. The acoustic viscosity sensor includes an acoustic wave generator, a transmitting piezoelectric transducer, a receiving piezoelectric transducer and a phase-shift detector. The acoustic wave generator, which is in contact with the fluid, is connected to the transmitting piezoelectric transducer. The transmitting piezoelectric transducer serves to transmit a longitudinal wave through the fluid. The longitudinal wave and a shear wave corresponding thereto are detected by the receiving piezoelectric transducer, which is spaced apart from the transmitting piezoelectric transducer and is connected to the phase-shift detector. The phase-shift detector is also connected to the transmitting piezoelectric transducer. The phase-shift detector detects a phase difference between the longitudinal wave transmitted by the transmitting piezoelectric transducer and the shear wave detected by the receiving piezoelectric transducer. The phase difference is used to obtain a velocity of the shear wave and a dynamic viscosity of the fluid. However, the acoustic viscosity sensor has a disadvantage in that the velocity of the shear wave is affected by the viscosity as well as a contaminant such as particles or bubbles in the fluid.
Recently, focus was directed to a viscosity sensor using a surface acoustic wave (SAW). The surface acoustic wave viscosity sensor is smaller than the acoustic viscosity sensor. Further, the surface acoustic wave viscosity sensor without the movable sensor part can conduct an in-situ measurement in a limited place or a poor environment. Especially, since the surface acoustic wave does not emit a large amount of energy into a fluid, it can detect the surface acoustic wave without excessive damping.
A measure principle of the surface acoustic wave viscosity sensor is directed to using the transfer of acoustic shear wave energy from a solid waveguide (e.g., a plate of quartz) having a characteristic material impedance Zw=(ρwμ)1/2 into an adjacent fluid having a characteristic material impedance ZL=(ωρLη)1/2. Herein, ρw is a density of the waveguide, ρL is a density of the fluid, μ is a shear elastic modulus of the waveguide, ω is a radian frequency of the acoustic shear wave, and η is a dynamic viscosity of the fluid. An energy transfer is proportional to the ratio ZL/Zw under the condition of ZL<<Zw. The square root of a power loss is proportional to the product ωρη of frequency, density and viscosity. If one knows the frequency, then the viscosity-density product can be measured. Further, although a turbulent flow occurs, the sample fluid remains stationary to ultrasonic vibrations in the quartz crystal regardless of the flow rate of the bulk fluid.
The surface acoustic wave viscosity sensor may be based on one of two modes, one of which wherein the wave propagates through a piezoelectric substrate and the other of which wherein the wave propagates on the piezoelectric substrate. Those modes are determined by a crystal cut of the quartz element. Methods using an acoustic wave in a thickness shear mode are disclosed in U.S. Pat. Nos. 4,741,200 and 5,741,961. Further, devices using a SH-APM (Shear-Horizontal Acoustic Plate Mode) and a FPW (Flexural Plate Wave) are disclosed in U.S. Pat. Nos. 6,304,021 and 7,287,431, respectively.
The surface acoustic wave viscosity sensor is small and has a microchip. However, the surface acoustic wave viscosity sensor is expensive due to expensive materials of the waveguide. Further, a depth δ=(η/πρ1ƒ)1/2, by which the acoustic wave is transmitted into the fluid in the surface acoustic wave viscosity sensor, is in inverse proportion to the square root of the frequency. Accordingly, there is a problem with the surface acoustic wave viscosity in that it locally responds to a physicochemical property of a minute region existing at an interface between the solid and the liquid.
The surface acoustic wave viscosity sensor operates at a frequency range of 1˜200 MHz. Further, when the acoustic wave with high frequency propagates through the fluid having a high molecular weight, a displacement rate of the fluid with the high molecular weight is smaller than a frequency at which the surface acoustic wave viscosity sensor vibrates. Thus, the fluid with the high molecular weight tends to behave like a gel. In such a case, the values measured by the surface acoustic wave viscosity sensor fail to accurately show the physicochemical properties under an actual usage of the fluid. Further, when contaminants exist in the fluid sample or aggressive agents exist in the fluid, particles may adhere on the surface of the surface acoustic wave viscosity sensor to thereby cause a distortion of measured signals. Thus, a sensitivity factor due to the contaminants or the aggressive agents must be considered beforehand. In addition, the surface acoustic wave viscosity sensor must be replaced regularly. Besides, the surface acoustic wave viscosity sensor using a high frequency inevitably has a complicated electronic circuit.
A magnetoelastic viscosity sensor is considered as an alternative to the surface acoustic wave viscosity sensor. The piezoelectric sensor generally uses a capacitive electrode, whereas the magnetoelastic viscosity sensor uses an inductive coil.
Recently, attention has been given to the magnetoelastic viscosity sensor using the inductive coil. The magnetoelastic viscosity sensor uses an amorphous metallic glass (Metglas) ribbon as its key part. The magnetoelastic viscosity sensor utilizes a phenomenon that a magnetoelastic ribbon vibrates in a length direction thereof when applying an alternating magnetic field to the magnetoelastic ribbon. A frequency of the alternating magnetic field is similar to a natural frequency of the magnetoelastic ribbon. The vibration generated from the magnetoelastic ribbon is measured as a voltage by a pick-up coil disposed around the magnetoelastic ribbon. A resonant frequency is proportional to the density and thickness of the magnetoelastic ribbon as well as other boundary conditions between the environment or fluid and a surface of the magnetoelastic ribbon. When a viscous fluid exists around the magnetoelastic ribbon, the resonant frequency is shifted towards lower frequency due to the dissipative character of the shear forces associated with fluid viscosity. The natural frequency may be obtained from the following Equation 1:
                                          Δ            ⁢                                                  ⁢            f                    =                                                    f                n                            -                              f                d                                      =                                                                                nf                    n                                                                    2                  ⁢                                      2                                    ⁢                                      ρ                    s                                    ⁢                  d                                            ⁢                                                (                                      ηρ                    L                                    )                                                  1                  2                                                                    ,                            Eq        .                                  ⁢                  (          1          )                    
wherein Δf is a frequency shift value, ρs is a density of the magnetoelastic ribbon, ρL is a density of the fluid, η is a dynamic viscosity of the fluid, and d is a thickness of the magnetoelastic ribbon.
That is, the resonant frequency of the magnetoelastic ribbon is proportional to the square root of the product of the viscosity and the density of the fluid. In Equation (1), the effect of the density ρL, of the fluid on the frequency shift value Δf may be similar to that of the aforementioned surface acoustic wave viscosity sensor. Thus, to measure the viscosity of the fluid independently, the density of the fluid must be known beforehand. However, a lubricant oil maintains its density almost constant (negligible change) in spite of a rapid change of the viscosity. Thus, the viscosity of such fluid can be measured immediately.
In this regard, a method of measuring an oil viscosity using the magnetoelastic ribbon is suggested in the following papers: a paper entitled “Magneto-acoustic sensors for measurement of liquid temperature, viscosity and density” (Jain, M. K.; Schmidt, S.; and Grimes, C. A.) published in the Applied Acoustics, Volume 62, Issue 8, August 2001, Pages 1001-1011; and a paper entitled “Effect of surface roughness on liquid property measurements using mechanically oscillating sensors” (Jain, M. K.; and Grimes, C. A.) published in the Sensors and Actuators, Volume 100, Issue 1, 15 Aug. 2002, Pages 63-69. FIG. 1 schematically shows an arrangement of a magnetoelastic ribbon according to the prior art. In the suggested methods, the magnetoelastic ribbon is located on a bottom of a glass beaker filled with oil. Referring to FIG. 1, the magnetoelastic ribbon 11 is placed on a plexiglas substrate 12. Baffle holes 13 with a diameter of 3 mm are formed in the substrate 12. Supports 14 for supporting the magnetoelastic ribbon 11 are disposed on the substrate 12. That is, the magnetoelastic ribbon 11 is spaced apart from the substrate 12 by the supports 14. Thus, it is possible to prevent the acoustic wave from reflecting again from a back side of the magnetoelastic ribbon 11.
Further, a method of decreasing an over damping, which occurs in a magnetoelastic ribbon when measuring a fluid viscosity using a thick magnetoelastic film sensor, is suggested by a paper entitled “Viscosity measurements of viscous liquids using magnetoelastic thick-film sensors” (Loiselle, K. T.; and Grimes, C. A.) published in the Review of Scientific Instruments 2000, Volume 71, Issue 3, Pages 1441-1446. FIG. 2 schematically shows a construction of a magnetoelastic sensor according to the prior art. Referring to FIG. 2, the magnetoelastic sensor 20 includes: a doughnut-shaped boat 21 immersed in an oil; a U-shaped bracket 22 mounted on the boat 21; a magnetoelastic ribbon 23 centrally disposed in the boat 21; and a coil 24 wound around the magnetoelastic ribbon 23. Insofar as a density of the oil remains constant, the magnetoelastic ribbon 23 may be immersed at a constant depth in the oil, although an absolute oil level changes. The magnetoelastic sensor 20 is immersed in the oil at a depth of 2.5 mm and measures a viscosity within a range of 100˜1500 cSt.
However, when the magnetoelastic sensor 20 is positioned in an oil tank, the magnetoelastic ribbon may float freely on the oil. Thus, the magnetoelastic sensor 20 cannot measure an accurate viscosity of the oil. Further, a power of an electromagnetic field cannot flow from the magnetoelastic ribbon 23 to the magnetoelastic sensor 20 in iron or ferromagnetic bodies. Accordingly, the magnetoelastic sensor 20 cannot be used in tanks, containers and pipes, which are made from an iron or ferromagnetic material. Furthermore, the magnetoelastic sensor 20 does not conduct measurement of the oil viscosity based on a change in the natural frequency of the magnetoelastic ribbon 23 according to a temperature change.